Effect of structure on enthalpy relaxation of polycarbonate: Experiments and modeling

The enthalpy relaxation of bisphenol A (BPA) and copolycarbonates of BPA and trimethylcyclohexylidene bisphenol (TMC) is studied using low heating rate DSC measurements. The glass transition behavior of the polycarbonates shifts to higher temperatures with increasing TMC content. The relaxation behavior of BPA is characterized by large and narrow peaks whereas the copolycarbonates exhibit a broader relaxation. In this work, the ability of the TNM model to describe the enthalpy relaxation behavior of the polycarbonate systems is tested and the sensitivity of the model parameters to the changes in chemical composition is examined. The model parameters are found to vary with thermal history and a linear correlation is observed between the nonlinearity parameter (x) and the nonexponentiality parameter (beta), although the nature of the parameter variation with thermal history is found to depend on the value of the activation energy parameter (Delta h/R). The results in this work indicate the need for reformulating the treatment of nonlinearity and incorporating a temperature-dependent activation energy in the model. (c) 2008 Elsevier Ltd. All rights reserved.